There are a wide variety of metal oxides and such metal oxides are used for various applications. Indium oxide is a well-known material and has been used as a transparent electrode material which is necessary in liquid crystal display devices or the like.
Some metal oxides have semiconductor characteristics. The examples of such metal oxides having semiconductor characteristics are tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. A thin film transistor in which a channel formation region is formed using such metal oxide has been already known (for example, see Patent Documents 1 to 4, Non-Patent Document 1, and the like).
As metal oxides, not only single-component oxides but also multi-component oxides are known. For example, InGaO3(ZnO)m (m is a natural number) having a homologous series is known as a multi-component oxide containing In, Ga, and Zn (also referred to as In—Ga—Zn-based oxide) (e.g. see Non-Patent Documents 2 to 4 and the like).
Furthermore, it is confirmed that an oxide semiconductor including an In—Ga—Zn-based oxide is applicable to a channel formation region of a thin film transistor (for example, see Patent Document 5, Non-Patent Documents 5 and 6, and the like).